Method of making a cemented carbide body with increased wear resistance

ABSTRACT

The present invention relates to a method of making a cemented carbide body with a bimodal grain size distribution by powder metallurgical methods including wet mixing, without milling, of WC-powders with different grain size distributions with binder metal and pressing agent, drying, pressing and sintering. The grains of the WC-powders are classified in at least two groups, a group of smaller grains and a group of larger grains. According to the method of the present invention, the grains of the group of smaller grains are precoated with a growth inhibitor with or without binder metal.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a reissue of U.S. Pat. No. 6,294,129 B1,filed on Jan. 13, 2000, which claims the benefit of priority to SwedishApplication No. 9900079 - 6 filed Jan. 14, 1999.

FIELD OF THE INVENTION

The present invention relates to cemented carbide bodies particularlyuseful in tools for turning, milling and drilling in steels andstainless steels.

BACKGROUND OF THE INVENTION

The following description contains references to certain compositions,articles, and methods. These references should not necessarily beconstrued as an admission that such compositions, articles and methodsqualify as prior art under the applicable statutory provisions.Applicants reserve the right to demonstrate that the below-describedsubject matter does not qualify as “prior art” against the claimedinvention.

Cemented carbide bodies are manufactured according to powdermetallurgical methods including milling, pressing and sintering. Themilling operation is an intensive mechanical milling in mills ofdifferent sizes and with the aid of milling bodies. The milling time isof the order of several hours up to days. Such processing is believed tobe necessary in order to obtain a uniform distribution of the binderphase in the milled mixture, but it results in a wide WC grain sizedistribution.

In U.S. Pat. Nos. 5,505,902 and 5,529,804 methods of making cementedcarbide are disclosed according to which the milling is essentiallyexcluded. Instead, in order to obtain a uniform distribution of thebinder phase in the powder mixture the hard constituent grains areprecoated with the binder phase, the mixture is further wet mixed withpressing agent dried, pressed and sintered. In the first mentionedpatent the coating is made by a SOL-GEL method and in the second apolyol is used.

Swedish patent application 9703738-6 discloses a method of producingsubmicron metal composite materials such as cemented carbide. Instead ofprecoating the WC grains with binder phase, the WC grains are precoatedwith elements inhibiting grain growth, such as Cr and V.

U.S. Pat. No. 5,624,766 discloses a coated cemented carbide insert witha bimodal distribution of WC grain size, with WC grains in two groups:0.1-1 μm and 3-10 μm. The insert according to this patent is producedwith conventional milling and sintering techniques resulting in aninevitable broadening of the WC grain size distribution during millingand grain growth during sintering.

WO 98/03690 discloses a coated cemented carbide insert with a bimodaldistribution of WC grain size, with WC grains in two groups: 0-1.5 μmand 2.56-6.0 μm. Although there is no milling, a certain amount of graingrowth takes place in the sintering step.

SUMMARY OF THE INVENTION

According to the present invention a method of making a cemented carbidebody with a bimodal grain size distribution comprises the steps of:

-   -   (i) wet mixing, without milling, WC-powders with a binder metal        and a pressing agent the WC powders comprising smaller grains        precoated with a grain growth inhibitor and larger grains;    -   (ii) drying the mixture of step (i);    -   (iii) pressing the dried mixture to form a pressed body; and    -   (iv) sintering the pressed body.

BRIEF DESCRIPTION OF THE DRAWING FIGURE

FIG. 1 shows in 1000× magnification of the cemented carbidemicrostructure according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

It has now surprisingly been found that improvement of the properties ofa cemented carbide according to U.S. Pat. No. 5,624,766 and WO 98/03690can be obtained if such a material is made using the coating techniquedisclosed in above mentioned Swedish patent application 9703738-6.Groups of smaller WC grains are precoated with grain growth inhibitors,with or without binder phase, and mixed with coarser hard constituentfractions which can be coated with binder phase according to any of thepreviously mentioned US patents. It is essential, according to theinvention, that there should be no change in grain size or grain sizedistribution as a result of the mixing procedure or as a result of thegrain growth in the sintering step. As a result a structurecharacterized of an extremely low grain growth is obtained.

According to the method of the present invention, a cemented carbidebody with a bimodal grain size distribution is made by powdermetallurgical methods including wet mixing, without milling, ofWC-powders with different grain size distributions with binder metal andpressing agent, drying, preferably by spray drying, pressing andsintering.

In preferred embodiments, the grains of the WC-powders are classified inat least two groups in which a group of smaller gains has a maximumgrain size a_(max) and a group of larger grains has a minimum grain sizeb_(min) wherein b_(min)−a_(max)>0.5 μm. It is further preferred that thevariation in grain size within each group is at least 1 μm, and thateach group contains at least 10% of the total amount of WC grains.

According to the method of the present invention the grains of the groupof smaller grains are precoated with a grain growth inhibitor.Preferably the grain growth inhibitor includes V and/or Cr, and thegrains of the group of larger grains are precoated with binder metal.The composition of the body comprises WC and 4-20 wt-% Co, preferably5-12.5 wt-% Co and <30 wt-%, preferably <15 wt-% cubic carbide such asTiC, TaC, NbC or mixtures or solid solutions thereof, including WC. TheWC grains are classified in two groups with a weight ratio of fine WCgrains to coarse WC grains in the range of 0.25-4.0, preferably 0.5-2-0.Preferably the two groups include the grain size ranges 0-1.5 μm (finegrains) and 2.5-6.0 μm (coarse grains).

In a one embodiment, the body is a cutting tool insert provided with athin wear resistant coating. Preferably the coating comprisesTiC_(x)N_(v)O_(z) with columnar grains followed by a layer of α-Al₂O₃,κ-Al₂O₃ or a mixture of α- and κ-Al₂O₃.

In a further embodiment, the W-content in the binder phase expressed asthe “CW-ratio” is 0.82-1.0, preferably 0.86-0.96 where the CW-ratio isdefined asCW-ratio=M_(s)/(wt-% Co*0.0161)

-   -   where M_(s) is the measured saturation magnetization of the        sintered insert in κA/m hAm² /kg and wt-% Co is the weight        percentage of Co in the cemented carbide.

EXAMPLE 1

A cemented carbide body with the composition, in addition to WC, of 10wt-% Co, and 0.3 wt-% Cr₃C₂ were produced according to the invention.Cobalt-coated WC with an average grain size of 4.2 μm, WC-3 wt-% Co,prepared in accordance with U.S. Pat. No. 5,505,902 and chromium coatedWC with an average grain size of 0.8 μm, WC-0.43 wt-% Cr, prepared inaccordance with 970378-6 was carefully deagglomerated in a laboratoryjetmill equipment, and mixed with additional amounts of Co to obtain thedesired material composition. The coated WC-particles consisted of 40wt-% of the particles with the average grain size of 4.2 μm and 60 wt-%of the particles with the average grain size of 0.8 μm, giving a bimodalgrain size distribution. The mixing was carried out in an ethanol andwater solution (0.25 liter fluid per kg of cemented carbide powder) for2 hours in a laboratory mixer and the batch size was 10 kg. Furthermore,2 weight-% lubricant was added to the slurry. The carbon content wasadjusted with carbon black to render a binder phase alloyed with Wcorresponding to a CW-ratio of 0.89. After spray drying, the insertswere pressed and sintered according to standard practice and a densebimodal structure with no porosity having an extremely low amount ofgrain growth was obtained.

FIG. 1 shows in 1000× magnification the cemented carbide microstructureformed according to this example.

EXAMPLE 2

A cemented carbide body with the composition, in addition to WC, of 10wt-% Co, and 0.3 wt-%-Cr₃C₂ were produced according to the invention.Cobalt-coated WC with an average grain size of 4.2 μm, WC-3 wt-% Co,prepared in accordance with U.S. Pat. No. 5,505,902 and chromium-cobaltcoated WC with an average grain size of 0.8 μAm, WC-0.43 wt-% Cr-2 wt-%Co, prepared in accordance with 9703738-6 was carefully deagglomeratedin a laboratory jetmill equipment, and mixed with additional amounts ofCo to obtain the desired material composition. The coated WC-particlesconsisted of 40 wt-% of the particles with the average grain size of 4.2μm and 60 wt-% of the particles with the average grain size of 0.8 μm,giving a bimodal grain size distribution. The mixing was carried out inan ethanol and water solution (0.25 liter fluid per kg cemented carbidepowder) for 2 hours in a laboratory mixer and the batch size was 10 kg.Furthermore, 2 weight-% lubricant was added to the slurry. The carboncontent was adjusted with carbon black to a binder phase alloyed with Wcorresponding to a CW-ratio of 0.89. After spray drying, the insertswere pressed and sintered according to standard practice and a densebimodal structure identical to Example 1 and with no porosity and havingan extremely low amount of grain growth was obtained.

EXAMPLE 3

A cemented carbide body with the composition, in addition to WC, of 10wt-% Co, 0.2 wt-% VC were produced according to the invention.Cobalt-coated WC with an average grain size of 4.2 μm, WC-3 wt-% Co,prepared in accordance with U.S. Pat. No. 5,505,902 and vanadium coatedWC with an average grain size of 0.8 μm, WC-0.28 wt-% V, prepared inaccordance with 9703738-6 was carefully deagglomerated in a laboratoryjetmill equipment, and mixed with additional amounts of Co to obtain thedesired material composition. The coated WC-particles consisted of 40.0wt-% of the particles with the average grain size of 4.2 μm and 60 wt-%of the particles with the average grain size of 0.8 μm, giving a bimodalgrain size distribution. The mixing was carried out in an ethanol andwater solution (0.25 liter fluid per kg cemented carbide powder) for 2hours in a laboratory mixer and the batch size was 10 kg. Furthermore, 2weight-% lubricant was added to the slurry. The carbon content wasadjusted with carbon black to a binder phase alloyed with Wcorresponding to a CW-ratio of 0.89. After spray drying, the insertswere pressed and sintered according to standard practice and a densebimodal structure identical to Example 1 and with no porosity having anextremely low amount of grain growth was obtained.

1. A method of making a cemented carbide body with a bimodal grain sizedistribution comprising the steps of: (i) wet mixing, without milling,WC-powders with a binder metal and a pressing agent, the WC powderscomprising smaller grains precoated with a grain growth inhibitor, andlarger grains; (ii) drying the mixture of step (i); (iii) pressing thedried mixture to form a pressed body; and (iv) sintering the pressedbody.
 2. The method of claim 1, wherein the smaller grains have amaximum size a_(max), and the larger grains have a minimum size b_(min)and wherein b_(min)-a_(max)>0.5 μm.
 3. The method of claim 2, whereinthe variation in grain size within each group of smaller and largergrains is at least 1 μm.
 4. The method of claim 1, wherein the smallergrains comprise at least 10% of the total amount of WC grains, and thelarger grains comprise at least 10% of the total amount of WC grains. 5.The method of claim 1, wherein the grain growth inhibitor is at leastone of V and Cr.
 6. The method according to claim 1, wherein the groupof larger grains are precoated with binder metal.
 7. The methodaccording to claim 1, wherein the composition of the mixture of step (i)comprises WC and 4-20 wt-% Co and <30 wt-%, cubic carbide comprisingTiC, TaC, NbC or mixtures or solid solutions thereof including WC. 8.The method according to claim 1, wherein in the WC grains beingclassified in two groups with a weight ratio of fine WC grains having asize of 0-1.5 μm to coarse WC particles having a size of 2.5-6.0 μm isin the range of 0.25-4.0.
 9. The method according to claim 6, whereinthe smaller grain size ranges from 0-1.5 μm and the larger grain sizeranges from 2.5-6.0 μm.
 10. The method according to claim 1, wherein thebody is a cutting tool insert.
 11. The method according to claim 10wherein the insert body is provided with a thin wear resistant coating.12. The method according to claim 11 wherein the coating comprisesTiC_(x)N_(v)O_(z) TiC_(x) N _(y) O _(z) with columnar grains followed bya layer of α-Al₂O₃, κ-Al₂O₃ or a mixture of α- and κ-Al₂O₃.
 13. Themethod according to claim 1, wherein the W-content in the Co binderphase expressed as the “CW-ratio” defined asCW-ratio=M_(s)/(wt-% Co*0.0161) where M_(s) is the measured saturationmagnetization of the sintered body in κA/m hAm² /kg and wt-% Co is theweight percentage of Co in the cemented carbide is 0.82-1.0.
 14. Themethod of claim 1, wherein step (ii) includes spray drying.
 15. Themethod of claim 1, wherein the precoating of the smaller grains of step(i) comprises binder metal.
 16. The method of claim 7, wherein thecomposition of the mixture of step (i) comprises WC and 5-12.5 wt. % Coand <15 wt. % of the cubic carbides.
 17. The method of claim 8, whereinthe weight ratio is in the range of 0.5-2.0.
 18. The method of claim 1,wherein only the smaller grains are precoated with the grain growthinhibitor.
 19. A method of making a cemented carbide body comprising thesteps of: (i) providing a WC powder, the WC powder comprises a group offine WC grains and a group of course coarse WC grains; (ii) precoatingthe fine WC grains with a grain growth inhibitor; (iii) precoating thecourse coarse WC grains with a binder metal; (iv) wet mixing, withoutmilling, the precoated fine WC grains, the precoated course coarse WCgrains, additional binder metal and a pressing agent; (v) drying themixture of step (iv); (vi) pressing the dried mixture to form a pressedbody; and (vii) sintering the pressed body.
 20. The method of claim 19,wherein steps (iv) and (vii) are performed such that no change in grainsize or grain size distribution are produced.
 21. The method of claim19, wherein the binder metal comprises Co.
 22. The method of claim 19,wherein the fine WC grains have a maximum size a_(max), the coarse WCgrains have a minimum size b_(min), and b_(min)-a_(max)<0.5 μm b_(min)-a_(max) >0.5 μm.
 23. The method of claim 19, wherein the fine grainscomprise at least 10% of the total amount of WC grains, and the coursegrains comprise at least 10% of the total amount of WC grains.
 24. Themethod of claim 19, wherein the grain growth inhibitor comprises atleast one of V and Cr.
 25. The method of claim 19, wherein the finegrains have a size of 0-1.5 μm and the coarse grains have a size of2.5-6.0 μm.
 26. The method of claim 25, wherein a weight ratio of fineWC grains to coarse WC grains is 0.25-4.0.
 27. The method of claim 26,wherein the ratio is 0.5-2.0.
 28. The method of claim 19, wherein thecemented carbide body has a CW-ratio of 0.82-1.0.
 29. The method ofclaim 28, wherein the CW-ratio is 0.86-0.96.
 30. The method of claim 13,wherein the CW-ratio is 0.86-0.96.